This invention relates to interferometers, e.g., displacement measuring interferometers for measuring displacements of a measurement object such as a mask stage or a wafer stage in a lithography scanner or stepper system.
Displacement measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object.
In many applications, the measurement and reference beams have orthogonal polarizations and different frequencies. The different frequencies can be produced, for example, by laser Zeeman splitting, by acousto-optical modulation, or internal to the laser using birefringent elements or the like. The orthogonal polarizations allow a polarizing beam splitter to direct the measurement and reference beams to the measurement and reference objects, respectively, and combine the reflected measurement and reference beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam, which subsequently passes through a polarizer. The polarizer mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a "heterodyne" signal having a beat frequency equal to the difference between the frequencies of the exit measurement and reference beams. If the lengths of the measurement and reference paths are changing relative to one another, e.g., by translating a stage that includes the measurement object, the measured beat frequency includes a Doppler shift equal to 2 .nu.p/.lambda., where .nu. is the relative velocity of the measurement and reference objects, .lambda. is the average wavelength of the measurement and reference beams, and p is the number of passes to the reference and measurement objects. Changes in the relative position of the measurement object correspond to changes in the phase of the measured interference signal, with a 2.pi. phase change substantially equal to a distance change L of .lambda./(2 np), where n is the refractive index of the medium through which the light beams travel, e.g., air or vacuum, and where L is a one-way distance change, e.g., the change in position of a stage that includes the measurement object.
Unfortunately, this equality is not always exact. Many interferometers include what are known as "cyclic errors," which are contributions to the phase of the measured interference signal and have a sinusoidal dependence on the change in optical path length 2 pnL. In particular, the first order cyclic error has a sinusoidal dependence on (4 .pi.pnL)/.lambda. and the second order cyclic error has a sinusoidal dependence on 2(4 .pi.pnL)/.lambda.. Higher order cyclic errors can also be present.
Cyclic errors can be produced by "beam mixing," in which a portion of an input beam that nominally forms the reference beam propagates along the measurement path and/or a portion of an input beam that nominally forms the measurement beam propagates along the reference path. Such beam mixing can be caused by ellipticity in the polarizations of the input beams and imperfections in the interferometer components, e.g., imperfections in a polarizing beam splitter used to direct orthogonally polarized input beams along respective reference and measurement paths. Because of beam mixing and the resulting cyclic errors, there is not a strictly linear relation between changes in the phase of the measured interference signal and the relative optical path length 2 pnL between the reference and measurement paths. If not compensated, cyclic errors caused by beam mixing can limit the accuracy of distance changes measured by an interferometer.